I like words that are several words shortened into one single word haha, like 깜놀 which means "what a surprise", or when you're shocked, and is a shortening of 깜짝 놀라다, and 엄친아 means mum's friend's son, meaning someone who's perfect, and it's a shortening of 엄마 친구 아들. I also like it when they say English words but in Korea, so konglish, like 레알 :D
찬열 사랑해 엑소 and kyeopta (cute) idk how to spell that in roman or korean xD OMO
My personal favorites are 죽을래 [jugeullae] (wanna die?) and 그래서 [geuraeseo] (so what?), but I caught myself saying 아이씨 [aissi] (popular "aish") and 아이고 [aigo] (you know that :P) ALL THE FREAKING TIME, I wonder what strangers think of me when I shout these things out loud...
Yah, Annyeong, Omo, Oppa, Eomma and Saranghae.
 aish, aigoo, jinjja, jen-jang ...  Usually said when something goes wrong for me!
"(I'm) screwed/(I) messed up". 망했다[manghaessda]/망했어[manghaesseo]. It cracks me up so hard when they say that xD Also 기가 막혀[kiga makhyeo] sounds so pretty ^_^ Finally, of course I love it when they call someone something with "saekki" or "imma".
aegisshi lol like u see i put it as my name too
michyeosseo,oppa,aigoo, omo omo
Anyoseo.....

I caught myself smilling while reading all your post..because i do the same, its just came out on lips whenever im with my friends..
Instead of calling by their names it just came out...YAH!!!!!.. and they think i was about to kick their butt..and even we have a serious conversation if i cant express what i want....Aissshhh!!!...Aigoo...ottoke...sometimes they think im an alien...
롤방 / Lolbang  - bun
물 / Mul or Bul - water

and in general I like all those words they use often^^
Heol
Yoengwonhee
Cheonsa
Words i use in everyday life (by accident ofcourse)

aigoo
aish
omo
chingu

i seem to go to CAPAY'S recently and have food the CHEP makes lol

jinjja? 
aigoooo  >.> 
Naega WAE?!! (why would I?) 
otteke ara??? 

bad romanization but oh well :P kenchanayo < another fav
Aish
Andwae
Babo
Heol
Jinjja
Ne (yes)
Ya!!!!
I love Korean language so much that even at home when there is dull moment I just speak Korean language and every body will start laughing. at home I am called a Korean lady even in the office lol, expressions I like are many such as:

Aiigooo, omo, sarayen, tunma, shiro, weeeey, etc